1. Field of the Invention
The present invention relates to a sheet bar joining apparatus for joining rough rolled sheet bars in continuous rolling equipment.
2. Description of the Related Art
In continuous rolling equipment for joining rough rolled materials (hereinafter referred to as sheet bars) to finish them continuously, it is generally necessary to join a rear end of a preceding sheet bar to a front end of a succeeding sheet bar, thereby increasing the work efficiency. A conventional sheet bar joining apparatus has joined the preceding sheet bar and the succeeding sheet bar together in the following manner: With the rear end of the preceding sheet bar and the front end of the succeeding sheet bar being clamped, these ends are heated with eddy currents generated by an alternating magnetic flux applied from an induction coil, and are pressed against each other in the heated condition. Such a sheet bar joining apparatus is disclosed, for example, in Japanese Unexamined Patent Publication No. 1005/95.
FIG. 7 is a plan view of the conventional sheet bar joining apparatus. FIG. 8 is a sectional view along line VIII--VIII of FIG. 7. FIGS. 9(a) and 9(b) are schematic views of an alternating magnetic flux applied from an induction coil in the conventional sheet bar joining apparatus, and eddy currents generated by the magnetic flux.
With the conventional sheet bar joining apparatus, as illustrated in FIGS. 7 and 8, a preceding sheet bar S.sub.1 and a succeeding sheet bar S.sub.2 are moved in a direction of an arrow A, as indicated in FIG. 7, by rolling rolls (not shown), with a rear end portion of the preceding sheet bar S.sub.1 and a front end portion of the succeeding sheet bar S.sub.2 being placed close to each other.
The succeeding sheet bar S.sub.2 can be clamped between an upper clamp 101 and a lower clamp 102 of an entry-side frame (not shown). Whereas the preceding sheet bar S.sub.1 can be clamped between an upper clamp 103 and a lower clamp 104 of a delivery-side frame (not shown). The clamps 101 and 102 of the entry-side frame can be moved in a direction of transport by a pressure cylinder (not shown), and thereby can press the front end portion of the clamped succeeding sheet bar S.sub.2 against the rear end portion of the preceding sheet bar S.sub.1.
The clamps 101 and 102 of the entry-side frame are provided with a pair of (i.e., upper and lower) anti-buckling plates 105 and 106 extending horizontally toward the delivery side. The anti-buckling plates 105 and 106 have upper comb tooth portions 107 and lower comb tooth portions 108, respectively. Heaters 109 are provided around the anti-buckling plates 105 and 106. The heater 109 comprises induction coils 112 and 113 provided so as to wind round upper and lower C-shaped iron cores 110 and 111.
By the action of the rolling rolls, the preceding sheet bar S.sub.1 and the succeeding sheet bar S.sub.2 are continuously transported, and the rear end portion of the preceding sheet bar S.sub.1 and the front end portion of the succeeding sheet bar S.sub.2 come to the center of the heater 109, with these end portions approaching each other. At this time, a hydraulic cylinder (not shown) is actuated to clamp the succeeding sheet bar S.sub.2 by the upper and lower clamps 101 and 102, and clamp the preceding sheet bar S.sub.1 by the upper and lower clamps 103 and 104. On this occasion, the sheet bars S.sub.1 and S.sub.2 are held in a sandwiched manner by the upper and lower anti-buckling plates 105 and 106 between the clamps 101 and 102 and the clamps 103 and 104. Thus, a difference in level between the sheet bars S.sub.1 and S.sub.2 is corrected, and their possible buckling is prevented.
In this state, a magnetic flux .PHI. is generated between the upper and lower C-shaped iron cores 110 and 111 to induce eddy currents E1 in the rear end portion of the preceding sheet bar S.sub.1 and the front end portion of the succeeding sheet bar S.sub.2, thereby heating these end portions to a temperature at which they can be joined together. At this time, eddy currents E2 are induced in the upper comb tooth portions 107 and the lower comb tooth portions 108. However, each of these eddy currents E2 must make a circle in a plane. Since its width is narrow, the circling adjacent eddy currents E2 interfere, counteracting each other. As a result, the current density decreases, and the Joule heat generated becomes small.
With the foregoing conventional sheet bar joining apparatus, the preceding sheet bar S.sub.1 and the succeeding sheet bar S.sub.2 are clamped, and held by the upper and lower anti-buckling plates 105 and 106 in a sandwiched fashion. In this state, as shown in FIGS. 7 and 8, the magnetic flux .PHI. is generated between the upper and lower C-shaped iron cores 110 and 111 to induce the eddy currents E1. The eddy currents E2 are also induced in the upper comb tooth portions 107 and the lower comb tooth portions 108. Under these actions, the sheet bars S.sub.1 and S.sub.2 are joined together.
In this case, at a widthwise central portion of each of the sheet bars S.sub.1 and S.sub.2, magnetic flux .PHI. passes through the upper comb tooth portions 107 and the lower comb tooth portions 108 as well as the sheet bars S.sub.1 and S.sub.2 to induce the eddy currents E2, as shown in FIGS. 8 and 9(a). Thus, the sheet bars S.sub.1 and S.sub.2 are heated at this site. At widthwise end portions of each of the sheet bars S.sub.1 and S.sub.2, on the other hand, magnetic reluctance is smaller when magnetic flux .PHI. flows outside the sheet bars S.sub.1 and S.sub.2 (i.e., magnetic flux .PHI..sub.1) rather than passing through the sheet bars S.sub.1 and S.sub.2, as shown in FIGS. 8 and 9(b). Thus, no heat elevation occurs at the widthwise end portions of the sheet bars S.sub.1 and S.sub.2.
This magnetic flux .PHI..sub.1 flowing outside the sheet bars S.sub.1 and S.sub.2 passes obliquely through end portions of the upper comb tooth portion 107 and the lower comb tooth portion 108. Thus, eddy currents E3 are induced in a vertical direction. Since the anti-buckling upper and lowerplates 105 and 106 have larger heights in the vertical direction than their widths in the plate width direction, these eddy currents E3 grow large. As a result, the comb tooth portions 107 and 108 are overheated to high temperatures. Hence, their fatigue increases, and their life shortens.
With the conventional sheet bar joining apparatus, moreover, when the rear end portion of the preceding sheet bar S.sub.1 and the front end portion of the succeeding sheet bar S.sub.2 are held by the upper and lower anti-buckling plates 105 and 106 in a sandwiched manner, the eddy currents E2 and E3 are induced in the comb tooth portions 107 and 108. These eddy currents E2 and E3 and the eddy currents E1 induced in the sheet bars S.sub.1 and S.sub.2 interfere with each other. Consequently, their sites of contact with each other spark, consuming the comb tooth portions 107 and 108.